1. Field of the Invention
The present invention relates to an electroacoustic transducer for converting an electric signal input thereto into sound.
2. Description of the Prior Art
An electroacoustic transducer is a means for converting an electric signal input thereto into sound. The electroacoustic transducer will produce an acoustic output in response to an input electric signal. Accordingly, the electroacoustic transducer can be employed by electronic devices, etc. as a sounding means such as a buzzer.
A prior art electromagnetic type electroacoustic transducer is shown in FIGS. 17-21 and includes a cylindrical outer casing 102 which is formed of synthetic resins and houses a magnetic driving portion 104 at the rear side thereof. Input terminals 106 and 108 are formed in the magnetic driving portion 104 for inputting an electric signal to the magnetic driving portion 104. The magnetic driving portion 104 has a columnar core 110 at the center thereof and a coil 114 is wound around the core 110 by way of a bobbin 112. The input terminals 106 and 108 are connected to the ends of the coil 114 which is energized in response to the electric signal input thereto by way of the input terminals 106 and 108. A cylindrical magnet 116 is provided on the inner wall of the cylindrical outer casing 102 and disposed about the coil 114.
A diaphragm 118, which is driven by the magnetic driving portion 104, is provided on the peripheral edge of the cylindrical magnet 116 and it is formed of an elastic thin magnetic member. Accordingly, the diaphragm 118 is attracted by the cylindrical magnet 116 and forms a closed magnetic circuit together with the core 110 and the cylindrical magnet 116. A magnetic piece 120 is attached to the center of the diaphragm 118 to establish a close magnetic relation with the core 110 and to add mass to the diaphragm 118.
At the front side of the diaphragm 118, there are provided a resonant chamber 122 which is closed by the cylindrical outer casing 102 and serves as a resonant space and a sound emitting cylinder 124 which permits the resonant chamber 122 to be open to the atmosphere. A plurality of ribs 126 for restricting the movement of the diaphragm 118 within an allowable moving range are provided on the wall surface of the resonant chamber 122 at the edge of the diaphragm 118.
FIG. 18 is an enlarged cross-sectional view of the diaphragm 118. The diaphragm 118 is formed of a very thin plate member and the disk-like magnetic piece 120 is attached to the center thereof as a rigid member to add mass to the diaphragm 118. The center of the magnetic piece 120 is attached to the center of the diaphragm 118 by spot welding. Denoted at 127 shows the welded portion.
It is necessary to sufficiently reduce the size of the welded portion 127 without deterioration of the characteristics of the diaphragm 118 so as to stabilize and make uniform the electroacoustic conversion characteristics. Furthermore, it is necessary that the deformation or deterioration of the characteristics of the diaphragm 118 is lessened after the diaphragm 118 and magnetic piece 120 are spot-welded and that they are brought into close contact with each other so as to have stable elasticity as a vibrating member. It is still necessary that the diaphragm 118 is very thin to assure a necessary sound pressure and sounding bandwidth.
If the sound pressure or the sounding bandwidth is increased, bonding strength between the diaphragm 118 and the magnetic piece 120 is decreased, which results in deterioration of reliability and stability of the electroacoustic transducer.
Meanwhile, such an electroacoustic transducer is provided in a variety of portable electronic devices and is subject to an external force such as a strong vibration, shock, etc. FIG. 19 shows the stationary state of the diaphragm 118 and FIG. 20 shows the vibrating state of the diaphragm 18 when it is normally driven. In FIG. 20, (a) shows the movement of the diaphragm 118 toward the core 110 and (b) shows the movement of the diaphragm 118 toward the sound emitting cylinder 124. That is, the diaphragm 118 repeats a vibration to thereby emit a sound depending on the frequency of the input electric signal. Normally, rated input and limited input level corresponding to the allowable moving range are set so that the diaphragm 118 is prevented from moving beyond the allowable moving range.
When an external force such as a shock, etc. is applied to the electroacoustic transducer, the diaphragm 118 is liable to be deformed beyond the allowable moving range as illustrated in FIG. 21. FIG. 21(a) shows the movement of the diaphragm 118 toward the core 110 wherein the diaphragm 118 contacts the head of the core 110. That is, the diaphragm 118 is prevented from moving excessively due to the core 110 so that the diaphragm 118 is protected by the core 110.
In the case as illustrated in FIG. 21(b) where an external force is applied to the electroacoustic transducer so as to push the diaphragm 118 upward toward the sound emitting cylinder 124, a stress is applied between the diaphragm 118 and the magnetic piece 120 in such a manner to tear the magnetic piece 120 from the diaphragm 118. As a result, there is a possibility that the diaphragm 118 is broken or deformed at the welded portion 127 or its peripheral portion. In case that the external force is strong, there is a possibility that the magnetic piece 120 falls out from the diaphragm 118.
There are measures for protecting the diaphragm 118 from the external shock as disclosed in Japanese Utility Model Publication No. 57-28478 entitled Electromagnetic Type Electroacoustic Transducer for Wristwatch, in Japanese Utility Model Laid-Open Publication No. 59-159098 entitled Electromagnetic Type Electroacoustic Transducer and in Japanese Utility Model Laid-Open Publication No. 60-26099 entitled Electromagnetic Type Sounder, etc. However, there are the following problems. In Japanese Utility Model Publication No. 57-28478, it is difficult to assure a resonant effect since a space in front of the diaphragm is sacrificed so as to prevent an excessive vibration. In Japanese Utility Model Laid-Open Publication No. 59-159098 and also in Japanese Utility Model Laid-Open Publication No. 60-26099, there remains a possibility that the magnetic piece falls out by the shock because the magnetic piece is not restricted in vibration.